1. Field of the Invention
The present invention relates to a battery control apparatus for controlling a battery carried by a hybrid vehicle.
This application is based on Japanese Patent Application No. Hei 11-242355, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, hybrid vehicles which carry electric motors as power sources for driving the vehicles in addition to engines are known. There are two types of the conventional hybrid vehicle: parallel hybrid vehicles and series hybrid vehicles. The series hybrid vehicle drives the motor by electric power generated by a generator which is driven by the engine, and the motor drives the wheels.
Since the engine and the wheels are not mechanically connected, the engine can be driven at an approximate constant engine speed and within an engine speed range in which the fuel consumption is reduced and the emissions are decreased. The series hybrid vehicle can reduce the fuel consumption and the emission as compared with the conventional vehicle.
The parallel hybrid vehicle assists the drive shaft of the engine by the motor connected to the engine, stores electric energy, produced by the motor which acts as a generator, in a power storage unit, and supplies the generated electric energy to other electrical equipment in the vehicle.
Although in the parallel hybrid vehicle the engine and the wheels are mechanically connected, the driving load on the engine is reduced, and the fuel consumption and the emission can be reduced as compared with the conventional vehicle.
In one type of the parallel hybrid vehicle, the motor is directly connected to the output shaft of the engine, and acts as a generator for charging a battery when the vehicle decelerates. In another type of parallel hybrid vehicle, either the engine or the motor, or both, generates the driving power, and a generator is separately provided.
These hybrid vehicles assist the output from the engine by the motor when the vehicles accelerates, charge batteries by deceleration regeneration when the vehicle decelerates, and perform various controls, to respond the driver's demands while maintaining the electric energy in the battery (hereinafter referred to as remaining battery charge).
The charging and discharging are repeated so that the remaining battery charge is maintained within a predetermined range (e.g., 20-80%) to protect the battery. This prevents over-charging and over-discharging of the battery, thereby lengthening the life of the battery and improving the efficiency.
However, there is the problem that the regenerated energy may not be efficiently accumulated in the battery because the upper limit of the charging is fixed at 80%.
Further, as the charging and discharging are repeated, while avoiding the over-charging and the over-discharging, so that the remaining battery charge is maintained within the intermediate range, the remaining charges of respective modules, which constitute the battery, may differ. Thus, the usable capacity of the battery may actually decrease, and the continuous operable time for assisting the output from the engine and for regenerating the energy may be shortened.